We hypothesize that "softly supported" bilayers incorporating selectins are excellent in vitro models for Surface Force Apparatus (SFA) measurements of the 'rolling forces' between complementary surfaces expressing selectins and its ligands. Isolating the molecular features (static and dynamic forces, mobility, on and off rates, etc.) of the specific ligands and receptors that control cell-cell and cell-substrate adhesion is difficult in vivo, although necessary for determining the basic rules governing cell sorting, tissue differentiation, pathological deformations and growths, and the body's control of cell population size and location. In addition, the dynamics of specific adhesion, i.e., of receptor -ligand pairs, is essential for understanding the body's immune response and in the biomimetic construction of drug delivery systems. The specific systems to be examined include: (1) Measurement and characterization of biomolecular and biosurface interactions. The Surface Forces Apparatus (SFA), AFM, and optical microscopy will be employed to measure bilayer, membrane and specific protein (e.g., ligand-receptor) interactions of softly-supported bilayers and monolayers. Specific systems will include: (1) the dynamic interactions of P-selectin proteins with various ligands such as lipid bilayers exposing polysaccharide headgroups (selectins are an important family of adhesion and recognition proteins), (2) DNA-lipid biolayer interactions which are primarily important for DNA transfection agents such as DNA- encapsulating vesicle delivery systems; (3) comparing these interactions to PEG-tethered biotin-lipids and streptavidin, this being a genetic tethered system with strong ligand-receptor bonds of varying tether length. (2) Biomimetic construction of complex structures. We plan to determine optimal methods of initiating, limiting, and controlling vesicle aggregation to produce aggregates of defined size and composition by manipulating (i) ligand-receptor stoichiometry, (ii) ligand-receptor "on- rates" through control of electrostatic or steric repulsions induced by pH, divalent ions and (iii) bilayer-bilayer and vesicle-vesicle interactions through control of surface charge or polymeric steric layers, ionic strength, or divalent ions. We also will study metastable bilayer structures for use as generic encapsulating membranes. The systems will include (i) mixtures of saturated phospholipids, cholesterol and polymer lipid that form open discs, and (ii) mixtures of long and short chain phosphatidylcholines, that form bilayer discs at low temperatures but close to form vesicles at higher temperatures, and (iii) ethanol interdigitated bilayers of DPPC that also form temperature dependent discs.